Cast iron articles can be manufactured with graphite morphology selected from a number of different shapes based on the mechanical, thermal, chemical, and tribological properties desired. Cast iron with flake graphite, called grey iron, exhibits excellent castability, good thermal conductivity, and excellent lubricity due to the graphite flakes. Cast iron with nodular graphite (also called spheroidal graphite) is generically called ductile iron due to its improved ductility over cast iron containing flake graphite. Ductile iron has better wear resistance and better tensile strength than cast iron with flake graphite. A third common graphite morphology is vermicular graphite (also called compacted graphite) which bridges nodular graphite and flake graphite. It is characterized by interconnected networks of thickened and rounded flakes within an iron matrix. Vermicular graphite cast iron (also called compacted graphite iron, CGI) has properties between that of ductile iron and grey iron.
In any given cast iron article it may be desirable to have some regions with properties associated with one of the graphite morphologies and another region with properties associated with another of the graphite morphologies. For example, in an engine block, it may be desirable to have better lubricity (associated with flake graphite) at a cylinder wall but to have better strength (associated with spheroidal graphite) in-between cylinders. For another example, it may be desirable to have better thermal conductivity (associated with flake graphite) in a cylinder head at the flame deck but better mechanical fatigue strength (associated with spheroidal graphite) on the outward-facing side of a cylinder head casting.
Prior art reference GB 1278031A discloses the concept of generating flake graphite at a cylinder wall while retaining spheroidal graphite elsewhere in a cylinder block. This reference generates the morphology difference by incorporating sulfur-containing compounds in the mold sand or on the surface of the mold that counteract nodularizing elements such as magnesium, calcium, or cerium in the molten cast iron. This technology is disclosed to be effective in modifying graphite morphology at the cylinder wall but the thickness disclosed as being affected was only 1.5 mm. While this reference does disclose that a deeper affected zone would be desirable (˜6 mm) no enablement was demonstrated.
Prior art reference JP 58-209443A discloses a similar treatment to mold sand as a means to generate flake graphite in the part. In addition to sulfur, the mold is covered with flake graphite; the disclosed affected depth is 0.5-3.0 mm.
Prior art reference JP 01-107958A discloses a mold that incorporates a region of low thermal conductivity; and, by using a cast iron with 0.012-0.02 wt % Mg, the region with slow cooling rate can be induced to form flake graphite. The thickness of the region with flake graphite is not disclosed. Prior art reference U.S. Pat. No. 4,807,728 discloses a related concept, but in that patent, the cast iron is a hyper-eutectic flaky graphite iron that is quickly cooled in one region; in the quickly-cooled region the graphite is much finer and substantially spheroidal.
Prior art reference JP 56-093851A discloses a cast iron article with regions of spheroidal graphite where the spheroidizing agent is placed directly in the mold at regions where the properties of spheroidal graphite are desired. The flake graphite regions are produced without any special treatment in the mold or in the molten iron.
Work conducted in conjunction with the present disclosure has shown that in-mold treatments of the type described in the prior art result in extensive mixing of the sulfurizing agent and thus are unable to produce localized graphite morphology control; despite what is disclosed in the prior art. In light of the persistent desire for localized graphite morphology control, an improved method for manufacturing cast iron articles with graphite morphology control is disclosed.